1. Field of the Invention
This invention relates to the production of semiconductor devices and, more particularly, to the production of such devices employing silicon.
2. Description of the Prior Art
A number of processing steps are required in the manufacture of a typical semiconductor device. Various levels of a device are formed in a succession of deposition steps. After these depositions, a semiconductor body, typically with a step-type geometry, is obtained. A common practice is to make the device planar by filling in these steps with an insulating material. Further, it is often desirable for this insulating deposited layer to act as a passivating layer (a layer which scavanges impurities either from the ambient atmospheres or from the contiguous semiconductor layers). To make the layer passivating, a dopant is added to the insulating material, e.g., silicon dioxide, during deposition. After the insulating layer is deposited as a final step in the preparation of the semiconductor device, a metal contacting layer, e.g., a layer of aluminum, is deposited on the insulating layer.
Many methods have evolved for the deposition of undoped silicon dioxide in the fabrication process previously described. For example, deposition mixtures of silane, carbon dioxide and hydrogen; silane and nitric oxide; or silane and nitrous oxide have been used satisfactorily. On the other hand, the deposition of doped silicon dioxide passivating layers presents problems unanticipated in the undoped silicon dioxide deposition methods.
A prime requisite for deposition of a passivating layer is that good step coverage is obtained. Several methods of depositing doped silicon dioxide layers have been developed in an attempt to satisfy this requisite. A first method entails the high temperature (typically 700 to 800 degrees C.) deposition reaction of an organo-silicon material with an organo-dopant material, e.g., organo-phosphorus or organo-boron compounds. As a result of the high temperatures used, the step coverage obtained is adequate. However, also due to the high temperatures used during the deposition process, silicon dioxide forms as particles on the walls of the reactor and subsequently falls into the structure being fabricated. These discrete silicon dioxide particles ultimately cause formation of pinholes in the final device which, in turn, severely degrade the obtainable electrical properties.
A low-temperature (typically 300 to 500 degrees C.) method has also been used for depositing a doped silicon dioxide layer. This method utilizes the reaction of silane, oxygen, and a dopant source, e.g., phosphine or diborane. The low temperature utilized, by itself, ensures pinhole free devices, but also yields poor step coverage. To improve the step coverage of this method an additional reflowing step is employed, i.e., the deposited, doped silicon dioxide is heated to a temperature typically between 1050 and 1150 degrees C.
Although reflowing improves the step coverage, it introduces further complications. The deposited, doped silicon dioxide layer does not readily melt in the reflowing step unless dopant concentrations greater than 7% are used. However, dopant levels exceeding 8% cause corrosion of the continuous aluminum contact layer through production of corrosive compounds, e.g., phosphorus dopants react with the moisture in the air to produce phosphoric acid.
Thus, if a reflow method is used, dopant concentrations in the silicon dioxide layer must be strictly limited to the narrow range between 7 and 8%. Because the dopant typically comprises no more than a few percent of the total gas flow used during deposition, and because it is difficult to control precisely the flow of the small quantities of dopants in the reaction mixture, it is very difficult to maintain this necessary concentration range in the deposited layer.